This invention relates to thermal spraying and more particularly to improved apparatus for shielding a supersonic-velocity particle-carrying flame from ambient atmosphere and an improved process for producing high-density, low-oxide, thermal spray coatings on a substrate.
Thermal spraying technology involves heating and projecting particles onto a prepared surface. Most metals, oxides, cermets, hard metallic compounds, some organic plastics and certain glasses may be deposited by one or more of the known thermal spray processes. Feedstock may be in the form of powder, wire, flexible powder-carrying tubes or rods depending on the particular process. As the material passes through the spray gun, it is heated to a softened or molten state, accelerated and, in the case of wire or rod, atomized. A confined stream of hot particles generated in this manner is propelled to the substrate and as the particles strike the substrate surface they flatten and form thin platelets which conform and adhere to the irregularities of the previously prepared surface as well as to each other. Either the gun or the substrate may be translated and the sprayed material builds up particle by particle into a lamellar structure which forms a coating. This particular coating technique has been in use for a number of years as a means of surface restoration and protection.
Known thermal spray processes may be grouped by the two methods used to generate heat namely, chemical combustion and electric heating. Chemical combustion includes powder flame spraying, wire/rod flame spraying and detonation/explosive flame spraying. Electrical heating includes wire arc spraying and plasma spraying.
Standard powder flame spraying is the earliest form of thermal spraying and involves the use of a powder flame spray gun consisting of a high-capacity, oxy-fuel gas torch and a hopper containing powder or particulate to be applied. A small amount of oxygen from the gas supply is diverted to carry the powder by aspiration into the oxy-fuel gas flame where it is heated and propelled by the exhaust flame onto the work piece. Fuel gas is usually acetylene or hydrogen and temperatures in the range of 3,000.degree.-4,500.degree. F. are obtained. Particle velocities are in the order of 80-100 feet per second. The coatings produced generally have low bond strength, high porosity and low overall cohesive strength.
High-velocity powder flame spraying was developed about 1981 and comprises a continuous combustion procedure that produces exit gas velocities estimated to be 4,000-5,000 feet per second and particle speeds of 1,800-2,600 feet per second. This is accomplished by burning a fuel gas (usually propylene) with oxygen under high pressure (60-90 psi) in an internal combustion chamber. Hot exhaust gases are discharged from the combustion chamber through exhaust ports and thereafter expanded into an extending nozzle. Powder is fed axially into this nozzle and confined by the exhaust gas stream until it exits in a thin high speed jet to produce coatings which are far more dense than those produced with conventional or standard powder flame spraying techniques.
Wire/rod flame spraying utilizes wire as the material to be deposited and is known as a "metallizing" process. Under this process, a wire is continuously fed into an oxy-acetylene flame where it is melted and atomized by an auxiliary stream of compressed air and then deposited as the coating material on the substrate. This process also lends itself to the use of other materials, particularly brittle ceramic rods or flexible lengths of plastic tubing filled with powder. Advantage of the wire/rod process over powder flame spraying lies in its use of relatively low-cost consumable materials as opposed to the comparatively high-cost powders.
Detonation/explosive flame spraying was introduced sometime in the mid 1950's and developed out of a program to control acetylene explosions. In contrast to the thermal spray devices which utilize the energy of a steady burning flame, this process employs detonation waves from repeated explosions of oxy-acetylene gas mixtures to accelerate powder particles. Particulate velocities in the order of 2,400 feet per second are achieved. The coating deposits are extremely strong, hard, dense and tightly bonded. The principle coatings applied by this procedure are cemented carbides, metal/carbide mixtures (cermets) and oxides.
The wire arc spraying process employs two consumable wires which are initially insulated from each other and advanced to meet at a point in an atomizing gas stream. Contact tips serve to precisely guide the wires and to provide good electrical contact between the moving wires and power cables. A direct current potential difference is applied across the wires to form an arc and the intersecting wires melt. A jet of gas (normally compressed air) shears off molten droplets of the melted metal and propels them to a substrate. Spray particle sizes can be changed with different atomizing heads and wire intersection angles. Direct current is supplied at potentials of 18-40 volts, depending on the metal or alloy to be sprayed; the size of particle spray increasing as the arc gap is lengthened with rise in voltage. Voltage is therefore maintained at the lowest level consistent with arc stability to provide the smallest particles and smooth dense coatings. Because high arc temperatures (in excess of 7,240.degree. F.) are encountered, electric-arc sprayed coatings have high bond and cohesive strength.
The plasma arc gun development has the advantage of providing much higher temperatures with less heat damage to a work piece, thus expanding the range of possible coating materials that can be processed and the substrates upon which they may be sprayed. A typical plasma gun arrangement involves the passage of a gas or gas mixture through a direct current arc maintained in a chamber between a coaxially aligned cathode and water-cooled anode. The arc is initiated with a high frequency discharge. The gas is partially ionized creating a plasma with temperatures that may exceed 30,000.degree. F. The plasma flux exits the gun through a hole in the anode which acts as a nozzle and its temperature falls rapidly with distance. Powdered feedstock is introduced into the hot gaseous effluent at an appropriate point and propelled to the work piece by the high-velocity stream. The heat content, temperature and velocity of the plasma gas are controlled by regulating arc current, gas flow rate, the type and mixture ratio of gases and by the anode/cathode configuration.
Up until the early 1970's, commercial plasma spray systems used power of about 5-40 kilowatts and plasma gas velocities were generally subsonic. A second generation of equipment was then developed known as high energy plasma spraying which employed power input of around 80 kilowatts and used converging-diverging nozzles with critical exit angles to generate supersonic gas velocities. The higher energy imparted to the powder particles results in significant improvement in particle deformation characteristics and bonding and produces more dense coatings with higher interparticle strength.
Recently, controlled atmosphere plasma spraying has been developed for use primarily with metal and alloy coatings to reduce and, in some cases, eliminate oxidation and porosity. Controlled atmosphere spraying can be accomplished by using an inert gas shroud to shield the plasma plume. Inert gas filled enclosures also have been used with some success. More recently, a great deal of attention has been focused on "low pressure" or vacuum plasma spray methods. In this latter instance, the plasma gun and work piece are installed inside a chamber which is then evacuated with the gun employing argon as a primary plasma gas. While this procedure has been highly successful in producing the deposition of thicker coats, improved bonding and deposit efficiency, the high costs of the equipment thus far have limited its use.
Related to the "low pressure" development is U.S. Pat. No. 3,892,882 issued Jul. 1, 1975 to Union Carbide Corporation, New York, N.Y. by which a subatmospheric inert gas shield is provided about a plasma gas plume to achieve low deposition flux and extended stand-off distances in a plasma spray process.
Aside from the few exceptions noted in the heretofore briefly described thermal spraying processes, all encounter some degree of oxidation of coating materials when carried out in ambient atmosphere conditions. In spraying metals and metal alloys, it is most desirable to minimize the pick-up of oxygen as much as possible. Soluble oxygen in metallic alloys increases hardness and brittleness while oxide scales on the powder and inclusions in the coating lead to poorer bonding, increased crack sensitivity and increased susceptibility to corrosion.